1. Field of the Invention
The present invention relates to a digital watermarking system, and more particularly to a digital watermarking system for making a digital watermark in an image signal.
2. Description of the Related Art
As demands for protection of the copyright of contents of image signals have been on the rise in today's computer age, and yet as an exemplified technology for embedding descriptive information in signals of the media, development of the technology called "digital watermarking" has been carried forward. Generally, watermarking for image is regarded as a technology of modifying the image signal or data in such a form as to be impossible or extremely difficult to recognize by the human's peculiar visual sense.
The digital watermarking technology is exemplified by International Laid-Open Patent No. WO95-14289 disclosing a concept of combining a certain number of noise screens distributed at random in space regions and superimposing the composite a noise screen with an original image.
Further, in their copending U.S. patent application Ser. No. 08/534,894, the present applicant has proposed a digital watermarking technology of transforming an input image into frequency regions, then modifying part of the spectrum to make a digital watermark and then reversely transforming the frequency regions into the image (space regions).
In this last-named technology, the original image is transformed into frequency domain data using Fourier formation or DCT (discrete cosine transform), whereupon a significant value in the resulting spectrum is selected. The significant value means a value influential on the original image when the data is modified or deleted; this significant value is discriminated using, for example, the absolute value of the spectrum as a reference.
The selected absolute value of the spectrum is multiplied by (1+.alpha.) where .alpha. is a random number whose upper limit is approximately 0.1; the value of this random number and the position of the spectrum jointly define a digital watermark. Then the modified spectrum is transformed by reverse Fourier transform or reverse DCT to obtain a digital-watermarked image.
This method merely intends to weaken a digital watermark down to such a level not to be recognized by the human's visual sense, utilizing the lengthy depth of luminance level of the image.
According to the conventional technology, if the original image has few colors like, for example, a map image, it is impossible to make a digital watermark in such image data if not modified. Yet if a digital watermark could be made in the unmodified image data, the original image would have been greatly collapsed.
If the format of the image is transformed to secure an adequate luminance level, e.g., if the image of total eight colors represented by 3 bits of RGB (Red, Green, Blue) is transformed into an 24-bit image represented by 8 bits each of RGB with the hue kept unchanged, it is possible to secure such a depth of luminance level that a digital watermark can be made in the image. However, it would be possible to assume the primary colors of the original image; that is, the digital watermark can be deleted as by performing a simple quantification of color.